Kuske, whose office has been flooded with calls from concerned patients, said this large cell lymphoma does not look or feel like breast cancer and is usually painless.

"Breast cancer shows up like a marble in front of the silicone implant. You can usually feel it," he explained. "This lymphoma shows up more as a thickening of the tissue or a hardening of what they think is the implant, but it's actually a rare white blood cell cancer in front of the implant -- a smooth thickening rather than a marble."

While doctors don't know for sure what causes the cancer, Kuske has a theory about how it develops. He said he believes the body senses that the implant is a foreign object and sends white blood cells to attack it.

"These white cells are very active, and if one of those cells gets too active, it can become cancer," he said.

Kuske said that while patients should not panic, they need to be informed and proactive.

"I think they need to do [regular] self-breast examinations," he said. "If they feel a change in their breast -- something that hasn't been there before -- present it to their doctor. It could be anywhere in the breast."

"We are very interested in trying to understand more specifically which patients may be at more risk and which breast implants may present a higher risk," said Dr. William Maisel, FDA's chief scientist for devices, on a call with reporters.

At this point, the FDA has not seen any difference in cancer rates between patients who have silicone implants, which were pulled off the market for years over safety concerns, and those who have saline implants. There was also no difference between those who got implants for cosmetic reasons and those who had reconstructive surgery after breast cancer.

The FDA is working with the American Society of Plastic Surgeons to register cancer patients so they they can be tracked over time.

While the lymphoma is fast-growing, doctors say it is often curable with radiation, chemotherapy or a bone marrow transplant.